1. Field of the Invention
This invention relates to a focus detecting apparatus suitable for an optical machine such as a camera, and in particular to a focus detecting apparatus for detecting the focus adjusted state of an objective lens.
2. Related Background Art
In single-lens reflex cameras or the like, a focus detecting system of the so-called image deviation type is known in which light distributions regarding two object images are formed from light beams from two different portions of the pupil of the photo-taking lens and the focus state of the photo-taking lens is detected from the relative positional relation between the light distributions.
FIG. 5 of the accompanying drawings depicts a longitudinal sectional shape of a single-lens reflex camera provided with a focus detecting unit of the image deviation type, and FIG. 6 of the accompanying drawings is a transverse sectional view in which only the optical system is developed and depicted to illustrate the focus detecting action.
In these figures, the reference numeral 1 designates an interchangeably mounted or fixed photo-taking lens, the reference numeral 3 denotes a field lens provided near the predetermined imaging plane 2 of the photo-taking lens 1 (the focal plane in a camera), the reference numerals 4 and 5 designate secondary imaging lenses disposed symmetrically about the optic axis L of the photo-taking lens 1 for forming two object images on the basis of light beams passing through different portions 1a and 1b of the pupil of the photo-taking lens 1, and the reference numerals 6 and 7 denote photoelectric conversion arrays for detecting the respective object images formed by the secondary imaging lenses 4 and 5. The arrays 6 and 7 are constructed, for example, of CCDs (charge coupled devices) or the like on a chip.
The reference numeral 8 designates a mask provided near the secondary imaging lenses. The field lens 3 causes the opening portions 8a and 8b of the mask 8 to be imaged on the different portions 1a and 1b on the pupil of the photo-taking lens 1.
The reference numeral 9 denotes a quick return mirror which is mirror-surface-treated on a light-transmitting substrate except for the aperture near the optic axis, and which is obliquely disposed in the photo-taking optical path during observation and is retracted out of the optical path during photographing. A pentaprism P and an eyepiece E are successively disposed on the reflecting optical path of the quick return mirror 9, and a sub-mirror M, the field lens 3, an optical path inverting mirror M2, the secondary imaging lenses 4 and 5 and the photoelectric conversion arrays 6 and 7 are successively disposed on the transmitting optical path of the quick return mirror 9. PC designates a signal processing circuit which calculates a value indicative of the focus adjusted state of the objective lens 1 on the basis of the signals of the arrays 6 and 7. F denotes the imaging plane on which a silver chloride film or an image pickup element is disposed.
In such an apparatus, when for example, the photo-taking lens 1 is moved leftwardly as viewed in FIG. 6 to provide the so-called forward focusing, light patterns, for example, object images, formed on the light-receiving surfaces of the respective photoelectric conversion arrays 6 and 7 by the secondary imaging lenses 4 and 5 deviate in the direction of arrow and thus, the fact of forward focusing and the amount thereof are detected from the variation in the outputs of the photoelectric conversion arrays 6 and 7 corresponding to the relative deviation of the pair of light patterns. Also, in the case of backward focusing, the respective images deviate in the direction opposite to that in the case of forward focusing and therefore, the fact of backward focusing and the amount thereof are detected.
FIG. 7 of the accompanying drawings depicts the optical action during in-focus in greater detail. The point of intersection between the optic axis L and the surface O of an object to be photographed is O1, and a point off the axis is O2. A light beam emitted from the point O1 is imaged on the photoelectric conversion arrays 6 and 7 by the action of the secondary imaging lenses 4 and 5, and the points at which the light beam is imaged on the arrays are P1 and Q1.
Paying attention to the light beam emitted from the point O2 off the axis, this light beam having a field angle is once imaged on or near the predetermined imaging plane, whereafter it is re-imaged on the photoelectric conversion arrays 6 and 7 by the secondary imaging lenses 4 and 5. The points at which the light beam is re-imaged on the arrays are P2 and Q2. The spacing Z1 between the points P1 and P2 and the spacing Z2 between the points Q1 and Q2 correspond to the images linking the points O1 and O2 together and must therefore be equal to each other.
However, as can be seen from FIG. 5, if an attempt is made to provide a focus detecting unit at the bottom of a camera, the light beam refracted and passed through the substrate of the quick return mirror 9 will be measured. If, at that time, the direction of arrangement of the secondary imaging lenses is made orthogonal to the direction of arrangement shown in FIG. 5, it means that as shown in FIG. 8 of the accompanying drawings, the quick return mirror 9 is disposed in the optical path while being inclined with respect to the direction of division D of the pupil of the photo-taking lens 1. The imaged states of the light beams passed through the portions 1a and 1b of the pupil differ from each other and therefore, even if the various aberrations of the optical system are corrected well, the position of the imaging plane which provides the reference differs depending on the field angle and the aforementioned spacings Z1 and Z2 become unequal. More specifically, as shown in FIG. 9 of the accompanying drawings, Z1-Z2 by the field angle exhibits an inclined characteristic and therefore, even in the case of objects at the same object distance, there arises an inconvenience that the discrimination of the focus state of the lens differs between the central area and marginal area of the distance measurement field.
Accordingly, in the case of an ordinary single-lens reflex camera, optical elements must avoidably be disposed so that the direction of division of the pupil becomes horizontal, and this has been a limitation in the optical arrangement.